Accuracy of Tach

[whistler66]

What is the easiest way to check a tach for accuracy? I'm really beginning to doubt mine. The dealer is 1.5 hours away and the traffic is a pain. Does someone sell a relatively inexpensive spark induction type device that has multiple settings (for # of cyl etc) so it's not a one time use thing and I can add it to my tool box and use it on other vehicles.

 

[Silvertip]

Re: Accuracy of Tach

An inductive tach does not have to be # of cylinders adjustable because it is reading only one cylinder. One pulse = one revolution on a two stroke. Two revs/pulse on a four stroke (unless it uses the wasted spark process where the plug fires at the bottom of the stroke as well.)

 

[robert graham]

Re: Accuracy of Tach

What kind of motor?, Why do you doubt it?.

 

[whistler66]

Re: Accuracy of Tach

An inductive tach does not have to be # of cylinders adjustable because it is reading only one cylinder. One pulse = one revolution on a two stroke. Two revs/pulse on a four stroke (unless it uses the wasted spark process where the plug fires at the bottom of the stroke as well.)

Just thinking out loud wouldn't the wasted spark of a 4 stroke be on the exhaust stroke of that piston/cylinder instead of firing at the bottom as you mentioned?
I was just trying to find the simplest thing to test or compare my current tach to. It seems like I had heard of an induction type gauge or something that I could use. I was trying to prevent having to hard wire anything in. Hoping to keep it simple.

What kind of motor?, Why do you doubt it?.

Yamaha 50HP 4stroke with an older OMC tach on board. Tach seems to be reading fast? I'm only basing this on previous experience with outboard engines. Both 4 stroke and 2 stroke BUT I may be off and the Tach be spot on!

 

[robert graham]

Re: Accuracy of Tach

Well,you know that motor is going to idle around 650 to 750RPM's, and max out at WOT around 5500RPM's, if set-up, propped correctly, and if motor is running right. If you're running WOT, trimmed up, down the lake, then tach should be showing about 5500RPM's, give or take a couple of hundred. So what's making you think it's not right? I've heard of a product called a Tiny Tach that folks use to tune motors but don't know much about them. Good Luck!

 

[whistler66]

Re: Accuracy of Tach

RG thanks for sticking with me on this. New motor, and I actually have only a couple of runs on the old tack with the old motor. So I really can't say it was ever right or wrong? Again just experience and sound of the Yamaha here. Couldn't find and idle number in my book and tach shows 1000 at idle. I thought it should be more in the neighborhood your talking about. 2,000 RPM by the tach is just barely moving the boat in no current water. Forgot my GPS yesterday to get a reading so any numbers would just be a guess. I didn't rig it so I don't know what the settings are on the rear of the tach. Got any guess as to what they should be and I'll give them a look? It say's OMC in small letters across the bottom. If need be I might get a photo on here.

 

[robert graham]

Re: Accuracy of Tach

Maybe take a small screwdriver and with motor running change settings on back of tach until readings appear what would be normal. 1000RPM's at idle is too high(if it's correct), not real good for shifting/gears. May need a new tach. Think I saw some at Walmart the other day.

 

[ajgraz]

Re: Accuracy of Tach

To really really know your engine's rpm, you are going to need an optical tach. You stick a bit of reflective tape on the flywheel, aim the optical tach's light at it, and it gives the flywheel rpms.

Any other method (like inductive) demands that every part of the ignition system is functioning 100% correctly...no missing spark, no switchbox problems, etc.